Plan 9 - man page for rendezvous (plan9 section 2)
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rendezvous - user level process synchronization
ulong rendezvous(ulong tag, ulong value)
The rendezvous system call allows two processes to synchronize and exchange a value. In
conjunction with the shared memory system calls (see segattach(2) and fork(2)), it enables
parallel programs to control their scheduling.
Two processes wishing to synchronize call rendezvous with a common tag, typically an
address in memory they share. One process will arrive at the rendezvous first; it sus-
pends execution until a second arrives. When a second process meets the rendezvous the
value arguments are exchanged between the processes and returned as the result of the
respective rendezvous system calls. Both processes are awakened when the rendezvous suc-
The tag space is common to processes in the same file name space, so rendezvous only works
between processes in the same file name space.
If a rendezvous is interrupted the return value is ~0, so that value should not be used in
The correlation of rendezvous tags and file name space is a historical accident. If two
unrelated processes happen to be in the same name space and do a rendezvous, trouble will
result. The solution is to call rfork(RFNAMEG) (see fork(2)) in programs that use ren-
dezvous unless they need to share the name space with their parent. This is especially
important in Alef programs.
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